1. Field of the Invention
The present invention relates to a removable water-borne urethane resin coating composition. More particularly, it relates to a removable water-borne urethane resin coating composition containing a polyvalent metal complex as a cross-linking agent.
2. Description of the Prior Art
A coating composition is used to coat a floor or the like and, followed by drying or other necessary processing to form a coating on the floor so as to preserve the attractive appearance of the floor surface, to prevent the floor from being stained and to protect the flooring material. The coating composition can be applied to various flooring materials including wood and synthetic resin.
Floor coatings for wooden floor have been used for a long time. The most common coating composition, a solution of a wax ingredient such as paraffin wax or carnauba wax dissolved in turpentine oil, is applied to the wooden floor surface, dried to a semi-dry state and then wiped with a dry cloth. However, the coating composition mainly composed of a wax has a disadvantage that the durability of the resultant coating is not satisfactory.
In order to provide a floor coating material which overcomes the aforementioned disadvantage, various synthetic resins developed by recent advances in chemical technology are dissolved in a suitable solvent, such as a thinner or organic solvents, to form a floor coating material, examples of such synthetic resins being polystyrene, acrylic resins, polyvinyl chloride, polyesters, epoxy resins, and copolymers thereof.
Remarkable progress has also been made in the floor materials which are coated. In ten years beginning from 1955, wooden flooring used in houses and office buildings were replaced by chemical flooring materials, which now account for 90% or more of the floor materials in use today. However, since the main ingredients of chemical flooring materials are synthetic resins such as polyvinyl-asbestos resin, polyvinylchloride resin or asphalt, such flooring materials tend to be dissolved in a base solvent of the floor coating compositions such as petroleum or naphthene base solvents and thereby are deteriorated. Moreover, because petroleum or naphthene base solvents may be toxic to workers and are flammable, the floor coating compositions dissolved in organic solvents have been gradually replaced by water-emulsion compositions.
Thus, synthetic resin emulsion type coating compositions have developed from around 1960, with many improvements being made to styrene resin emulsions, styrene-acrylate copolymer resin emulsions and acrylate resin emulsions.
We have been investigating water-borne polyurethane resin coating composition with the aim of preparing a floor coating material having properties superior to those of the aforementioned emulsion type coating compositions, and have found that commercial water-borne polyurethane resins cannot be used directly as a floor coating compositions for the following reasons.
A floor surface coated with any composition, even a coating composition with superior durability, is inevitably galled, stained and abraded by the wear and tear of being walked on, and inevitably the coating surface is tinged with yellow or otherwise deteriorated with the lapse of time. In consideration of this, it becomes necessary to remove old coatings and apply a new coating in order to preserve the beautiful appearance or the floor and to protect the floor.
Although the coating formed by applying a commercial water-borne polyurethane resin on the floor surface, followed by complete curing is excellent in adhesiveness, gloss and toughness, it has the disadvantage that it is difficult to remove from the floor surface using chemical means. If such a coating is removed using a strong solvent, the chemical flooring material is also dissolved by the solvent. On the other hand, the flooring is damaged if the coating is removed by mechanical grinding.
We have therefore made various studies to prepare a coating composition having the properties itemized in the following (1) to (3) while maintaining the excellent properties of water-borne polyurethane resin.
(1) Easy to apply to a variety of materials, particularly on a floor, to form a coating which is tough and durable; PA1 (2) Slight staining on the coating can be removed by the use of a weak alkaline cleaning agent; PA1 (3) If the stains penetrate deep into the coating surface or the coating becomes yellowish with the lapse of time, the coating can be cleaned or removed by the use of a strong alkaline cleaning agent containing ammonia or an amine.
It has been found that the requirements set forth above are satisfied by the provision of a polyurethane resin containing carboxyl groups or carboxylate groups with the addition of a polyvalent metal complex as a metallic cross-linking agent. The present invention is based on the above finding.